1. Field of the Invention
The present invention relates to a lighting power supply device that performs dimming by using a phase-controlled dimmer, and particularly to a light emitting diode (LED) lighting power supply device capable of controlling dimming of a lighting device that uses the LED.
2. Description of Related Art
In recent years, for the purpose of reducing an amount of carbon dioxide emission, a lighting fixture (hereinafter, referred to as an LED lamp) provided with an LED that consumes less electric power has been becoming widespread in place of an incandescent lamp that consumes much electric power. Heretofore, in a power supply device for the LED lamp, there has been proposed a technique for controlling dimming by using a phase-controlled dimmer (see Japanese Patent Application Laid-Open Publication No. 2007-227155 as a Patent Literature 1). Moreover, also in a power supply device for the incandescent lamp, such dimming control that uses the phase-controlled dimmer is performed.
As disclosed in the Patent Literature 1, the power supply device for the LED, which includes the phase-controlled dimmer, is composed of: a phase-controlled dimmer including a thyristor or a triac as a switching element receiving an alternating current power supply voltage from a commercial alternating current power supply, and including a control unit that controls to switch on/off the switching element; a rectifying circuit that converts an alternating current into a direct current; a lighting power supply circuit having an AC-DC converter that supplies desired electric power to the LED lamp; and the like. In the phase-controlled dimmer, an ON-phase angle of the switching element is controlled by the control unit in response to a resistance value of a variable resistor as dimming adjusting means, and the like, whereby a duty ratio of the alternating current power supply voltage is changed, and the dimming of the LED connected to the lighting power supply circuit is performed.
In such an LED lighting power supply device using the phase-controlled dimmer, it is necessary to control brightness of the LED lamp based on an input signal as shown in FIG. 8A. Therefore, desirably, in a control circuit of such an LED lighting power supply device, a time-voltage conversion circuit is provided, which generates a time-voltage conversion signal with a rectangular waveform as shown in FIG. 8B, the time-voltage conversion signal having a pulse width corresponding to a phase, from the input signal as shown in FIG. 8A, and converts the pulse width of the conversion signal concerned into a voltage, and then an LED drive current is controlled based on the voltage subjected to such time-voltage conversion.
As the time-voltage conversion circuit, for example, a circuit as shown in FIG. 9 is conceived. The circuit of FIG. 9 is composed of: a comparator CMP that generates, from the input signal of FIG. 8A, such a pulse signal as shown in FIG. 8B, the pulse signal corresponding to the phase; a CR filter FLT that is provided on a subsequent stage to the comparator CMP and generates a direct current voltage corresponding to the pulse width; and a voltage follower VF that performs impedance conversion for the generated voltage and transmits the voltage concerned.
However, though plural dimmers in which variable ranges (variable angles) of adjusting knobs (operation dials) differ from one another are present as dimmers provided at present on the market, the time-voltage conversion circuit shown in FIG. 9 can only generate a direct current voltage determined uniquely in response to the pulse width since a time constant of the CR filter is fixed. Therefore, in a lighting system using the time-voltage conversion circuit as shown in FIG. 9, there is a problem that only dimming control corresponding to such a variable range of the adjusting knob can be performed. Moreover, there is a problem that a response to a phase change of the input signal is slow since the CR filter is used.